Author: szeloof

Preface

I am very excited to announce the details of my first integrated circuit and share the journey that this project has taken me on over the past year. I hope that my success will inspire others and help start a revolution in home chip fabrication. When I set out on this project I had no idea of what I had gotten myself into, but in the end I learned more than I ever thought I would about physics, chemistry, optics, electronics, and so many other fields. Furthermore, my efforts have only been matched with the most positive feedback and support from the world; I owe a sincere thanks to everyone who has helped me, given me advice, and inspired me on this project. Especially my amazing parents, who not only support and encourage me in any way they can but also give me a space to work in and put up with the electricity costs… Thank you!

Without further ado, I present the first home(garage)made lithographically-fabricated integrated circuit – the “Z1” PMOS dual differential amplifier chip. I say “lithographically-fabricated” because Jeri Ellsworth made the first transistors and logic gates (meticulously hand wired with conductive epoxy) and showed the world that this is possible. Inspired by her work, I have demonstrated ICs made by a scalable, industry-standard, photolithographic process. Needless to say, this is the logical step-up from my previous replication of Jeri’s FET fabrication work. 

Design

I designed the Z1 amplifier looking for a simple chip to test and tweak my process. Layout was done in Magic VLSI for a 4 mask PMOS process (active/doped area, gate oxide, contact window, and top metal.) PMOS has advantages over NMOS as far as mobile ionic contamination that lends it to being fabricated in a garage. The masks are designed in 16:9 aspect ratio for easy projection.

The feature (gate) size is approximately 175μm although there are test features as small as 2μm on the chip. Each amplifier section (center and right) contain 3 transistors (2 for long-tailed differential pair and one as current source/load resistor) which means a total of 6 FETs on the IC. The left portion of the IC contains resistors, capacitors, diodes, and other test features used to characterize the fabrication process. Each node of the differential pairs is broken out to a separate pin on the lead frame so it can be analyzed and external biasing can be added as necessary.

EDIT: see update at the bottom, the transistor gate length has been reduced to <5µm (1975 tech. level) which brings an increase in device performance.

Fabrication

There are 66 individual fabrication steps to make this chip and it takes approximately 12 hours for a full run. The process yield can be as high as 80% for these large features, but is largely dependent on my coffee intake that day. I have also made Youtube videos covering semiconductor fabrication theory and discrete MOSFET fabrication.

50mm <100> orientation Silicon wafers with bulk resistivity 1 to 10 Ω-cm (30.8 to 308 Ω/sq for thickness of 325µm) are scored into 5.08 x 3.175mm dies (~16mm^2 area) with an Epilog fiber laser. Polyvinyl Alcohol in water or photoresist can be spun on the wafer prior to laser scribing to “catch” laser ablation debris and the film is later removed in solvent before processing. This die size is chosen to fit into a Kyocera 24pin DIP carrier.

Native oxide is stripped off the wafer with a quick dilute HF dip and then they are extensively cleaned in Piranha solution (H2SO4:H2O2), RCA 1 (H2O:NH3:H2O2), RCA 2 (H2O:HCL:H2O2), followed by another dilute HF dip. Most of these cleaning dips are for 10 minutes and can be facilitated by raising to ~40ºC.

The field oxide is thermally grown in a water vapor ambient (wet oxidation) to a thickness of 5000-8000Å. One may consider mixing the DI water for this step with a few percent HCl. The Chrloine atoms help getter and immobilize ionic contaminants and are also said to increase the growth rate by 5-7%. Together with the fact I am making PMOS devices rather than NMOS, these give a huge edge over contamination control and allow decently preforming devices to be fabricated in a garage.

The oxidized wafer is ready for patterning of the active/doped (P-type) area. Positive photoresist (AZ MiR 701 for SiO2 patterning and AZ 4210 for Al layer) is spun on at around 3000rpm yielding a film of about 1.5μm for the AZ MiR 701 or 3.5μm for the AZ 4210 which is soft baked at 90C on a hotplate.

Lithography process details

The active area mask is exposed with my Mark IV maskless photolithography stepper at 365nm UV and the pattern is developed in TMAH or KOH solution depending on the resist.

The resist pattern is then hard baked and a number of other tricks are used to ensure good resist adhesion and chemical stability during the following HF etch step which transfers that pattern to the oxide layer and opens windows to the bare silicon surface for doping. These regions later become the source/drain of the FETs.

Doping is then carried out by either solid or liquid source. The solid source is a Boron Nitride disk that is placed in proximity (<2mm) from the wafer in the tube furnace. Alternatively, spin-on liquid sources can be prepared from Phosphoric or Boric acid in water or solvents and doping is carried out in a standard pre-deposition/HF dip/drive-in/deglaze process. I obtained Phosphoric acid in pure form on Amazon and Boric acid from Roach & Ant killer. Since the starting wafer for PMOS here is N-type, I am doing P diffusions of Boron for the source/drain regions and am targeting a sheet resistance in diffused regions of 100 to 250 Ω/sq.

The above mentioned patterning steps are then repeated twice for the gate oxide layer and then the contact layer. The gate oxide must be much thinner (<~750Å) than the field oxide, so the regions between the source/drain are etched away and a thinner oxide is grown there. Then, since the whole wafer has been oxidized during the doping step, contact windows must be etched for the metal layer to make connection with source/drain doped regions.

 

(click to enlarge)

Now, all the transistors are formed and are ready to be interconnected and broken out to the lead frame. A blanket layer of Aluminum (400-500nm) is sputtered or thermally evaporated onto the wafer. An alternative would be to use the lift-off process in which the photoresist is patterned first and then metal is deposited. To support wire bonding, this metal layer is made thicker (around 2.5µm for Au wire wedge bonding.) These films have a measured bulk resistivity around 5.4e-6 Ω-cm for thermally evaporated films, double the ideal value of 2.7e-6 Ω-cm for Al at 20ºC. The incorporation of Oxygen and other gasses into the Al film during vacuum deposition likely accounts for this difference.

The metal layer is then patterned with photolithography and etched in hot Phosphoric acid (50ºC) to yield the completed IC. The final steps before testing are visual inspection and high temperature annealing of the Aluminum to create ohmic connections.

The finished chip is now ready for packaging and testing.

Testing

I don’t have a wire bonder (accepting donations!) so my testing right now is limited to manually probing the wafer with sharp tweezers or using a flip-chip board (difficult to align) to connect it to a curve tracer. The differential amplifier is also tested empirically in-circuit to verify operation.

EDIT: see update at the bottom, I now have a wire bonder!

As you can see above in the PMOS FET Id vs. Vds curves, there is lots of die to die variation and devices made on the same day can have widely different characteristics. Taking 5 traces with -1V Vgs increment requires about a -8V body/substrate bias to overcome fixed charges (positive impurity ions trapped under gate) and lattice defects in the gate region and yield the expected graph.

Chips can be tested easily and repeatably by probing or wire bonding.

Electrical characteristics of Al/Si junctions are characterized as well and show the expected results. We can create three such basic contacts between Aluminum and Silicon. Aluminum is P-type with respect to Silicon so Schottky diodes are formed whenever Aluminum comes into contact with lightly doped N Silicon. Sometimes my devices showed a tunneling characteristic rather than the expected diode, so I theorize that if the same device is processed for a longer time under high temperatures (>1000ºC), increased oxidation at the Si surface causes the Phosphorous at the surface of the wafer to “pile-up” because of the increased solubility of N-type dopant in SiO2. This creates an “N+” region at the surface and the higher dopant concentration creates a diminishing depletion layer which relates to a small potential energy barrier (the electrons can easily tunnel across it), explaining the symmetrical IV curve.

Additionally, the gate oxide dielectric breakdown voltage can be destructively tested. For high quality SiO2, this should be a little over 1V/nm and is easily tested by sweeping Vgs up from 0V and noticing when a large current flows (in normal operation the gate is insulating and no current should be able to flow).

This plot shows gate dielectric breakdown occurring at 21.7V for a 25nm thick gate device, indicating a decent thermally grown oxide quality which could be improved by being grown in an atmosphere with higher Nitrogen content.

The switching and differential amplifier characteristics can also be demonstrated. The trace on the right shows the output of the chip configured as a fully differential amplifier, mixing (adding/subtracting) a 1kHz and 50kHz sine waves together.

The final characteristic to test for is a low-leakage, fully insulating gate as one of the main requirements of true MOSFET operation. As you can see, I am able to charge up the gate of the device and turn it on through a high impedance connection through my fingertip, and the 1, 0 states of the FET are “latching” due to charge staying on the gate of the FETs and having no pathway to dissipate.

Update 9/3/18: I got a wire bonder (K&S Al/Au wedge bonder)! It will definitely take some more practice before I can bond to a chip, but results will be posted. This will also allow for more extensive testing. I just moved out to college so progress will hopefully be made on school breaks. A huge thanks to Jeremy Gordon (@JeremySF on Twitter) for the gracious donation.

Update 7/8/19: FET gate length (feature size) reduced to <5µm, brining this project to be state-of-the-art in about 1975 and allows the transistors to operate with much better characteristics. Pictured below is a ~4.5µm Aluminum gate and the corresponding characteristics showing 5 traces, -3V step and a -8V body/substrate bias.

 

Thanks for following my work and feel free to contact me with your thoughts at sam@zeloof.xyz !

In IC fabrication it is necessary to deposit/grow and etch insulating layers of Silicon Dioxide. This presents a few problems for standard photolithographic patterning because SiO2 is hydrophilic which can cause photoresist adhesion issues and also the HF etchant attacks most photoresists. These issues combine to leave you with poor pattern definition and often complete photoresist lifting during etch.

The steps I have found to mitigate these issues are (in order): dehydration bake, HMDS vapor prime, thick resist coating, hard bake, and buffered oxide etch.

First, SiO2 is thermally grown on a test wafer using a water vapor source on a nearby hotplate to fill the furnace with steam during oxidation. The first step to ensure good resist adhesion is a dehydration bake which creates a hydrophobic wafer surface. This does not need to be done if the wafer recently came out of the furnace but if it has been in storage, then a bake of up to 700C may be necessary to restore the dehydrated surface. The next step is HMDS vapor priming: 

Here, the wafer is heated to around 200C in the presence of Hexamethyldisilazane (HMDS) vapor forming a surface monolayer on the wafer that further increases resist adhesion. HMDS can also be spin coated but this often yields a far too thick layer and can lead to incomplete photoresist development.

The final steps before etch are to spin the resist and to hard bake it. Naturally, a thicker resist film allows for a longer etch time. For maximum chemical stability, the hard bake should be conducted for extended periods of time close to the resist softening point which is usually around 145C. This can make the photoresist difficult to remove, so an ultrasonic acetone bath may be necessary unless you have proper stripping chemicals. If difficulty persists, then it is likely that the top layer of resist has cross-linked and you may be unable to remove it. One may try high power Oxygen RIE followed by Piranha solution and N-Methylpyrrolidone (NMP) stripper as is used commercially to remove resists after hard ion implantation.

Instead of a standard HF etch, a buffered oxide etch of NH4F (Ammonium Fluoride) in HF can be used to control the etch rate and photoresist lifting. I use approximately 20-30g of 100% NH4F per 50mL of HF (stock whink rust remover) and etch time for 6000Å SiO2 is 20min at 20C. A couple drops of Triton X-100 nonionic surfactant may be added to the BOE to improve etch uniformity, wetting, and ensure consistency through a thicker resist. A good BOE recipe can be found here but assumes industrial-strength HF.

Etch trials

Details and photos of my newest maskless photolithography stepper have been posted here: Info Page

Automated DLP submicron stepper for 2″ (50mm) wafers with LabView control, computer alignment, and wafer vacuum chuck. Based on an old Nikon microscope with custom optics and in-situ UV-VIS spectroscopy for illumination process control. Diffraction-limited resolution is <250nm with a 365nm light source.

Lift-off is a technique that allows you to patten a metal layer without any etchant chemicals. Photoresist is spun on, exposed, and developed then the metal is sputtered or evaporated on top of the resist. The photoresist is then striped and any metal on top of it is peeled off. This leaves metal in only the areas in which the resist was not present after developing.

Typically, negative photoresist is preferred for lift-off for a number of reasons but I did not want to change my existing process so I attempted it with positive AZ4210 resist yielding decent results.

Photoresist can be removed in acetone or developer solution (assuming it is exposed by ambient light during processing) but a lower vapor-pressure solvent is preferred. An ultrasonic bath can also improve lift-off.

If lift-off is difficult, a thicker resist film or thinner metal coating can help. Also, substrate heating during deposition leads to resist softening and side wall coating, making lift-off impossible. I used a Peltier cooler in my chamber to prevent this.

Using a 50W Ytterbium fiber laser to scribe and cut Silicon wafers. Laser wavelength is 1062nm and since Silicon has poor transmission at this low IR wavelength, enough energy is absorbed to make laser marks.

Scribe marks are made at higher speed and must follow perpendicular or parallel to a flat of the <100> wafer so that the scribe lies along a crystal lattice line. When cutting all the way through the wafer, higher power is used and arbitrary shapes that do not follow the crystal lines are possible.

Presentation about home semiconductor fabrication at the 2018 Trenton Computer Fest

Thanks to all who attended!

There are a number of open source layout and design tools for ICs however here I will just focus on Magic VLSI. Below are the steps to take a design from Verilog through synthesis and layout to the physical mask that is used for fabrication, via the Qflow digital synthesis flow.

The Verilog digital design for this example is a UART interface from www.asic-world.com and is synthesized/routed with the standard SCMOS rules, however a custom .tech file can be specified containing process details and design rules for DRC.

Once Qflow executes successfully, the design can be viewed in Magic after loading the appropriate cells and a GDS file is generated. This is opened in OwlVision GDSII Viewer or similar which can be used to generate the individual mask image files for each layer (active, poly, metal, etc.)

Using the same tools mentioned above, I also designed a simple PMOS chip to test my process. It is scalable to any reasonable size and contains 2 differential amplifier circuits (seen on right and middle) and a number of diodes/resistors and other test features on the left.

The design requires 4 masks for fabrication: active/diffusion, gate oxide, contact, and metal.

Fiducials should be added for subsequent layer alignment. Note: Grateful Dead bears are necessary for the circuit to function correctly.